Among structures of bacterial iron-containing proteins, functional in photosynthesis and nitrogen fixation, a number available in pure form and bulk quantities have been crystallized and found adequate for tertiary structure determinations at high resolution, using a variety of procedures including x-ray crystallographic analysis, and a wide range of modern spectrochemical methodology (ORD, CD, NMR, EPR and IR spectroscopy). These preparations include the variant cytochromes c (cytochrome C2, cytochrome c', and "HIPISP") of a number of purple photosynthetic bacteria, cytochromes "c-555" of green photosynthetic bacteria, and the algal cytochromes "f" of red alga. Complete amino acid sequences for these proteins have been determined. Using our preparations, high resolution x-ray determinations of tertiary structure have been reported in studies by associated groups for R. rubrum cytochrome c2 (reduced and oxidized) and of reduced Chromatium "HIPISP." Others are now being analyzed preparatory to extension of these structure studies to provide a detailed molecular basis for a comparative biochemistry of iron-containing proteins.